This article describes a tool that makes forging precisely matching field-frame rings very easy. It can also be used for forging washer rims. Making oval rings is very easy using this template, unlike with the old type template. That said, the old type template is still somewhat more useful for forging the washer cores.
The process step by step Edit
First find a piece of thick steel (5-10 mm) to use as a (sort of) anvil. It will only take glancing blows from the sides, so it doesn't have to be overly strong. Next drill a hole of suitable thickness (~10mm) through the middle of the plate.
The template itself is made from thick steel about 1.2 - 1.5 times thicker than the finished rings; this allows forging the ends of the (flattened) rings around the template so that they overlap. For cheiroballistra parts ~8mm steel is quite satisfactory. If the form of the template is non-trivial, i.e. not a circle, it's useful to print the outline on thick paper and punch it to the plate through the paper. Remember to punch a mark for the center hole, too:
The outline of the template should now be clearly visible on the steel plate:
Cut the plate to roughly correct form:
Finish the template by grinding the sides to correct curvature and drilling a hole through the center mark.
The parts are locked together using a steel rod going through the template and the anvil plate. The steel rod is sunk deep into a block of wood. The end result should look like this: